MOD 5   LECTURE 5                                     REFRACTION

In this lecture you will:

SUMMARY OF WAVE - MATTER INTERACTIONS
Type of waves Main interaction  SOUND LIGHT (EMS)
INTERFERENCE Run into each other acoustics (loud/soft) holograms (bright/dark)
REFLECTION Hit and bounce off surfaces acoustics: echoes and reverberations mirrors, colors, shiny and matte
REFRACTION Enter clear materials and bends rumble of thunderstorms lenses, diamonds, rainbows
DIFFRACTION Bend around solid or little slit how we hear around a post slits, rainbow effect on CD or DVD

 
REFRACTION: IN GENERAL
When waves enter transparent or translucent materials and bends

"Refraction is the bending of waves when they enter a medium where their speed is different. Refraction is not so important a phenomenon with sound as it is with light where it is responsible for image formation by lenses, the eye, cameras, etc. But bending of sound waves does occur and is an interesting phenomena in sound"

An example is of rolling wheels on a cart. When the leading wheels hit the grass which has more friction and drag, the leading wheel is slowed down first compared with the other wheel. This yanks the whole wagon toward the "normal". 

 Try this: if you are on roller skates, roll off the sidewalk with your right skate hitting the grass first and see if that pulls you around to the right.



 
SOUND REFRACTION
"Thunder is the acoustic shock wave resulting from the extreme heat generated by a lightning flash. Lightning can be as hot as 54,000°F (30,000°C), a temperature that is five times the surface of the sun! When lightning occurs, it heats the air surrounding its channel to that same incredible temperature in a fraction of a second.

Like all gases, when air molecules are heated, they expand. The faster they are heated, the faster their rate of expansion. But when air is heated to 54,000°F in a fraction of a second, a phenomenon known as "explosive expansion" occurs. This is where air expands so rapidly that it compresses the air in front of it, forming a shock wave similar to a sonic boom. Exploding fireworks produce a similar result."
 
The Rumble of Thunder citation

"When lightning strikes a shock wave is generated at each point along the path of the lightning bolt. (The above illustrations show only four points.)"

"With nearby lightning strikes the thunder will sound like a loud bang, crack or snap and its duration will be very short."

"As the shock wave propagates away from the strike center, it stretches, diminishes, and becomes elongated. Then other shock waves from more distance locations arrive at the listener."

"At large distances from the center, the shock wave (thunder) can be many miles across. To the listener, the combination of shock waves gives thunder the continuous rumble we hear."

"the temperature of the atmosphere affects the thunder sound you hear as well as how far away you can hear it. Sound waves move faster in warm air than they do in cool air. Typically, the air temperature decreases with height. When this occurs, thunder will normally have an audible range up to 10 miles (16 km).

However, when the air temperature increases with height, called an inversion, sound waves are refracted (bent back toward the earth) as they move due to their faster motion in the warmer air. Normally, only the direct sound of thunder is heard. But refraction can add some additional sound, effectively amplifying the thunder and making it sound louder."

"This is more common in the winter as thunderstorms develop in the warm air above a cooler surface air mass. If the lightning in these "elevated thunderstorms" remains above the inversion, then most of the thunder sound also remains above the inversion. However, much of the sound waves from cloud-to-ground strikes remain below the inversion giving thunder a much louder impact."


 
LIGHT REFRACTION
RATING OF REFRACTION: THE INDEX OF REFRACTION
Every time a wave of light exits one material and enters another material with a different "index of refraction", it will bend. 

It bends at both the entrance surface of the material and at the exit of the material as it passes into another material of different index of refraction.

A perfect vacuum has an index of refraction = 0.  The index of refraction for most materials > 1. (The symbol ">" means "greater than"). 

- Diamond has a high index (2.42) compared with air (1.00029).

-- the index of refraction is a measure of the optical density of the material. 

- The HIGHER THE INDEX of refraction, the HIGHER THE OPTICAL DENSITY. The HIGHER THE INDEX of refraction, the GREATER THE BEND.

-- the speed of light slows down when it enters any kind of material. The HIGHER THE INDEX of refraction, the SLOWER THE SPEED OF LIGHT.

-- there are two examples of why light bends in denser material. In the top example, they state that if the incident ray enters at "normal" or 90o (perpendicular), then all parts of the wave are slowed at the same time. However, when the incident ray enters at an angle, the edge that hits the slower (denser) material "drags" the wave (ray) toward the "normal".

-- when the ray exits into faster (less dense) material (like air), the leading edge speeds up and bends away from the "normal".

 



 
 

 

Summary:

The higher the index of refraction, The greater the wave bends

The higher the index of refraction, The greater the density of the Material

The higher the index of refraction, The slower the wave or the slower the SOL!

When a wave moves from less to More dense it bends toward the normal

When a wave moves from more to Less dense  it bends away from The normal

Perpendicular -No bending Lower to higher- bend toward
Faster to slower
Less dense to more dense
Higher to lower - bend away                          Slower to faster                                                  More dense to less dense

 
TRANSPARENCY: ABSORPTION, REFRACTION AND REMISSION
Transparent materials

–  various wavelengths of the EMS can pass thru different materials.
-  materials like water, glass, crystal, are transparent to visible waves, but not necessarily to other waves of the EMS.  For example, UV can go thru short distances of water, not thru glass, but can go thru crystal

WHAT DOES "GO THROUGH" MEAN?

1. when waves move thru transparent materials, it acts more like a particle or packet of energy than a "wave"

2. the packets or photons are absorbed by molecules of material on one side of the material, the energy wave is passed molecule to molecule until it emerges on the other side and is emitted.

3.  Light slows down below the "speed of light" when it moves thru transparent materials

4.  When light hits the surface square on, perpendicular, it slows as it goes thru, but doesnt bend.  In all cases, when light emerges on the other side it resumes the "speed of light". 


 
REFRACTION: PRISM
A prism splits light into component wave lengths by sending white light thru two REFRACTION events the second at an angle and the "double" bend splits white light. 

 
REFRACTION AND REFLECTION: RAINBOWS
"Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind at a low altitude angle. The most spectacular rainbow displays happen when half of the sky is still dark with raining clouds and the observer is at a spot with clear sky in the direction of the Sun. The result is a luminous rainbow that contrasts with the darkened background."

A raindrop breaks white light up by an initial refraction event and then reflects the light against the inside of the raindrop so the light bounces out the front of the drop where it bends again as it exits the raindrop.  This is two refractions and one reflection. To see a rainbow, the Sun needs to be at about 23o angle above the horizon. Stand with your back to the sun to see the rainbow. 

In water droplets the light experiences TOTAL internal reflection so the light bounces back out at the observer. 

This is a double rainbow. 

Notice how the area inside the inner rainbow is LIGHT, this is due to the raindrops refracting the light so it goes toward the CENTER.  Also notice that the shortest waves, blue, bend the most and appear on the inner aspect of the inner rainbow and red, the longest wavelength is on the outside.  The outer rainbow is reversed so that red is on the inside, blue on the outside. 

REFRACTION AND REFLECTION: DIAMONDS
"98% of a diamond’s fire and brilliance is created by the quality of its cut and proportions, not its clarity, color, or size. Its proportions, polish, and the symmetry of its facets are primary to its radiance. None of the other factors, color, clarity, or carat weight, have more of a dramatic effect on a diamonds beauty than the cut... Subtle and minute structural differences of half a millimeter or less will determine whether a diamond will be full of Brilliance or dull and lifeless." cite
Diamonds in the rough
It is both the prism like effect of two refractions and the internal reflections that make diamonds sparkle. Diamonds are good candidates because they have such a high index of refraction. 

"Bright diamonds return lots of light from the surroundings back to a 'face up' an observer. If light from above leaks out the back of a diamond, naturally it has less brightness. But light that enters and leaves in the face up direction is wasted because your head blocks the lights. Diamonds that are too deep or very shallow do this -they have areas that act like a mirror back to the viewer; they return less light and so they have less brightness. Fire or dispersed light appears as flashes of rainbow colors."

REFRACTION AND REFLECTION: SUN AND MOON PATHS
Water both REFRACTS and REFLECTS light. 

When the moon (or Sun) is at 48.75  (OK, 49o) degrees above the horizon a point called the "critical angle" is seen as a "path" of moon on water, or, a path of the Sun is seen. 

Early in the morning and late at night the angle is at around 49o all the Sun is reflected.  One reason drivers going east in the morning and west after work find the sun so blinding, the situation that drivers from the western suburbs have going to work in Milwaukee is because early and late the Sun is totally reflected right into their eyes. 
 
OTOH, if you are snorkeling it is best to go when the sun is 90o - 40o or so since the most light is refracted into the water.  

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SUMMARY OF EFFECTS
loud and soft areas of sound in concerts sound interference
bright or dim areas lined up in a row on a surface light interference
holograms  light interference using lasers
reverberation sound reflection
echoes sound reflection
mirrors light reflection
colors  light reflection and absorption
shiny or matte light reflection and interference
soap bubbles and oil spill rainbows light reflection and interference
the rumble of thunder sound refraction
prism rainbow light refraction
rainbows light refraction and reflection
diamonds twinkle light refraction and reflection
blinding early morning light  reflection less than critical angle 50o
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